1. Field of the Invention
The present invention relates to a thermal head having heat generators arranged on a substrate and driving integrated circuits (often called "IC" hereinafter) arranged on the substrate to drive the heat generators.
2. Description of the Prior Art
A thermal head is used for the thermal printing method in which color recording is performed on a heat recording paper by Joule heat generated when a pulsating current is applied to heat generators or an ink of a heat-sensitive ribbon is transferred by this Joule heat to a plain paper to effect recording.
Recently, in order to make the structure of the thermal head compact, not only heat generators but also driving IC elements are loaded on one substrate, and thermal heads of this type are now spread.
When a thermal head of this type is assembled, input and output electrodes on driving IC elements should be connected to a wiring pattern comprising heat generator driving lilnes connected to heat generators arranged on a substrate and external connection terminals, IC driving power lines and IC control signal lines. At the present, this connection is performed by wire bonding, and therefore, the following problems arise.
(1) Even if an automatic wire bonding apparatus is used for this wire bonding, respective electrodes of the driving IC elements should be wire-bonded to the wiring pattern one by one in sequence. Since about 0.2 second is necessary for wire-bonding one electrode, when a thermal head for B4 size comprising 32 IC elements, each having 84 input and output electrodes, at a density of 8 dot/mm with a printing width of 256 mm is assembled, about 10 minutes (0.2 second.times.84 electrodes.times.32 elements) are necessary for completing the wire bonding operation. Accordingly, mass production is difficult and the cost of the thermal head is increased. Moreover, the automatic wire bonding apparatus is expensive and the equipment cost is greatly increased.
(2) Before the wire bonding operation, driving IC elements should be die-bonded onto the substrate at predetermined positions. Furthermore, in this case, IC elements should be insulated from a wiring pattern on heat generator driving lines and the like formed on the substrate, and therefore, an insulating layer should be formed on the IC element-loaded portion for insulation from the wiring pattern. Accordingly, additional processes are necessary and increase of the cost owing to increase of the number of operation steps cannot be avoided.
(3) Since the bonding wire line is very fine and the diameter is 25 to 35.mu., bending or falling is readily caused after the bonding operation, and therefore, a short circuit is formed between input electrodes of the IC element. Consequently, it is impossible to increase the wire bonding density.
(4) Because of characteristics of the wire bonding method, the bonding pad portion of the wiring pattern on the substrate should be arranged outside the IC elements, and a broad bonding area is necessary around the IC chip-loaded portion. Because of this problem as well as the above-mentioned problem (3), it is more difficult to make the structure of the thermal head compact.